1. Field of the Invention
The present invention relates to a switching power supply that is used for an electronic device and the like, and more particularly, to a switching power supply device capable of obtaining a stable startup operation of a power source.
2. Description of the Related Art
There has been proposed a switching power supply device that controls an on/off operation of a switching element to control an output voltage is used for an OA device, a living device and the like. Recently, from standpoints of environments and energy saving, improvement of efficiency is required for the switching power supply device. In order to suppress loss of a switching element in the switching power supply device, a voltage resonance or current resonance is used. A control circuit that controls the resonance operation is typically comprised of an integrated circuit of one chip.
FIG. 6 is a circuit diagram showing a structure of a related-art switching power supply device 2b. The switching power supply device 2b is a pseudo resonance-type switching power supply device. The switching power supply device 2b has an alternating current power source 1, a bridge rectifier DB, a smoothing condenser C1, a transformer T, a switching element Q1, a current detection resistance R1, a rectification diode D1, an output condenser C2, an error amplifier 4, photo-couplers PCa, PCb, a condenser C9, a condenser C10 for an auxiliary power source, a diode D10 and a controller 3b for controlling the switching element Q1, as shown in FIG. 6.
The controller 3b has, as external input terminals, a startup terminal (DV terminal) that is connected to a drain terminal of the switching element Q1, an input terminal (Vcc terminal) of a power source, a feedback signal input terminal (FB terminal), an over-current protection terminal (OC terminal), a voltage detection terminal (ZC terminal) of an auxiliary coil D of the transformer T, a DR terminal for outputting a control signal to the switching element Q1 and a ground terminal (GND terminal) of the controller 3b. 
The transformer T has a primary coil P, a secondary coil S and an auxiliary coil D and transfers the energy to a secondary side circuit. In addition, the switching element Q1 is connected to the primary coil P of the transformer T.
The error amplifier 4 is connected between Vout and SG and controls current flowing in the photo-coupler PCa in accordance with a difference between the output voltage Vout and an internal reference voltage. The photo-coupler PCa is comprised of a light emitting diode and feeds back an error for a reference voltage to a primary side. In addition, the photo-coupler PCb is a photo transistor that operates in accordance with the light from the light emitting diode of the photo-coupler PCa and has a collector connected to the FB terminal of the controller 3b and an emitter grounded to the GND terminal.
An auxiliary power source for the controller 3b is structured in such a way that the diode D10 and the condenser C10 are connected to the auxiliary coil D, rectifies/smoothes voltage induced to the auxiliary coil D of the transformer T and charges the voltage in the condenser C10 of the auxiliary power supply to supply power to the Vcc terminal of the controller 3b. 
The voltage that is induced to the secondary coil S during the off period of the switching element Q1 is rectified/smoothed by the rectification diode D1 and the output condenser C2 and then outputted to a load from Vout, as a secondary side output voltage.
In addition, as shown in FIG. 6, the controller 3b includes a startup circuit StartUp, an internal power source Reg, logic circuits NOR1, OR1, comparators BD, F, OCP, a flip flop circuit FF1, resistances R4, R5, R6, a diode D3, reference voltages Vz, Voc and a drive circuit BF.
The internal power source Reg starts up the controller 3b based on power supplied from the Vcc terminal and supplies power required for the whole operation of the controller 3b. In addition, the startup circuit StartUp supplies power to the internal power source terminal Vcc till a predetermined voltage in inputting the power and stops the supply after oscillation of the controller 3b starts, so that it is switched to an auxiliary power source obtained by rectifying the voltage from the auxiliary coil D of the transformer T.
The voltage Vreg of the internal power source Reg generates a feedback voltage from the secondary side to the FB terminal by the photo-coupler PCb and the condenser C9 which are provided outside of the controller 3b and are connected to the FB terminal.
The voltage Vreg of the internal power source Reg is connected to the ground GND via the resistance R4, the diode D3, the resistance R5 and the resistance R6 and the FB terminal is connected to the resistance R4 and an anode of the diode D3.
In addition, an inverting terminal (minus (−) input terminal) of the comparator F is connected with the resistance R5 and the resistance R6 and is applied with a voltage that is proportional to the FB terminal voltage.
The OC terminal is connected to a source terminal of the switching element Q1 and the resistance R1, is applied with a voltage depending on the current flowing in the switching element Q1 and outputs a voltage signal to a non-inverting terminal (plus (+) input terminal) of the comparator F and a non-inverting terminal of the comparator OCP.
The comparator F outputs a H signal when a voltage signal depending on the current flowing in the switching element Q1, which is outputted from the OC terminal of the controller 3b, exceeds a voltage Vfb of an input terminal. Thereby, when the voltage value of the OC terminal voltage signal exceeds the voltage value vfb depending on the feedback amount from the secondary side appearing in the FB terminal, the comparator F inputs a signal of an H level to an S terminal of the flip flop circuit FF1 through the OR circuit OR1 and turns off the switching element Q1 through the logic circuit NOR1 and the drive circuit BF, thereby controlling the output voltage of the secondary side at a constant value.
When the voltage signal outputted from the OC terminal exceeds a reference voltage value voc, the current flowing in the switching element Q1 becomes an over-current. Thus, the comparator OCP outputs an H signal.
When the H signal is inputted by any one of the comparator OCP and the comparator F, the logic circuit OR1 outputs the H signal to the S terminal of the flip flop circuit FF1.
A non-inverting terminal of the comparator BD is connected to the ZC terminal, and the ZC terminal is connected to the auxiliary coil D of the transformer T through the resistance R3. An inverting terminal of the comparator BD is connected to a reference voltage Vz and an output terminal of the comparator BD is connected to an R terminal of the flip flop circuit FF1 and one input terminal of the logic circuit NOR1.
The comparator BD compares a fly-back voltage of the auxiliary coil D with the reference voltage Vz, completes the energy discharge of the accumulated energy of the transformer T to the secondary side through the secondary coil S and detects that a polarity of the coil voltage is inverted. At the time at which the coil voltage of the auxiliary coil D is lowered below the reference voltage Vz, the output terminal of the comparator BD outputs an L signal to the R terminal of the flip flop circuit FF1 and the logic circuit NOR1 and switches the switching element Q1 to an on state from an off state through the drive circuit BF.
The flip flop circuit FF1 outputs a control signal from a Q terminal, based on the signal inputted to the S terminal and the signal inputted to the R terminal. The Q terminal of the flip flop circuit FF1 is connected to one input terminal of the logic circuit NOR1. In addition, an output of the logic circuit NOR1 is connected to the drive circuit BF. The switching element Q1 is on-off controlled in accordance with an output of the logic circuit NOR1.
In the followings, an operation of the related-art switching power supply device 2b will be described. First, the sinusoidal voltage outputted from the alternating current power source 1 is rectified in the bridge rectifier DB, which then passes through the smoothing condenser C1 and is outputted to the drain terminal of the switching element Q1 through the primary coil P of the transformer T.
The switching element Q1 is turned on/off by the controller 3b and each coil of the transformer T is supplied with the energy, so that the current flows in the secondary coil S and the auxiliary coil D.
The current flowing in the secondary coil S is rectified/smoothed to become direct current power by the diode D1 and the output condenser C2, which is then outputted to an external load from Vout.
As the on/off operation of the switching element Q1 is repeated, the output voltage of Vout is gradually increased. When the output voltage reaches a reference voltage set in the error amplifier 4, the current flowing in the photo diode of the photo-coupler PCa is increased. Thus, since the current flowing in the photo transistor of the photo-coupler PCb is increased, the condenser C9 is discharged and the voltage of the FB terminal is lowered. Thereby, as described above, the controller 3b controls the switching element Q1 through the comparator F, the logic circuit OR1, the flip flop circuit FF1, the logic circuit NOR1 and the buffer circuit BF, thereby stabilizing the output voltage of Vout.
The current flowing in the auxiliary coil D rectified/smoothed by the diode D10 and the condenser C10, so that it is used as an auxiliary power source of the controller 3b and supplies power to the Vcc terminal. As described above, when the Vcc terminal once reaches the startup voltage, the power supply from the startup circuit StartUp is cut off. Accordingly, the power supply to the Vcc terminal after the startup is carried out by the auxiliary power source circuit including the auxiliary coil D, the diode D10 and the condenser C10. Since a polarity of the auxiliary coil D is the same as that of the secondary coil S, the voltage of Vcc is proportional to the output voltage of Vout.
Herein, regarding the switching operation of the switching element Q1 from an off state to an on state, a ringing waveform of the transformer is used which is generated after the power discharge of the secondary coil S of the transformer T. In other words, the switching element Q1 is turned on to a bottom of the ringing waveform of the auxiliary coil D of the transformer.
Further, in order to prevent a malfunction, measures may be taken in which some time is provided so as to prevent the switching element Q1 from being again turned on by the ringing just after the turn off or off time is prolonged so as to reduce a switching loss under light load, thereby making a low switching frequency. For example, such technology is disclosed in JP-A-2002-315330.
An oscillation frequency of the above-described pseudo resonance-type ringing choke converter is often set to be about 20 kHz in order to improve efficiency and avoid audible frequency within a range of the input voltage or load conditions, even though the oscillation frequency is varied depending on the input voltage and load conditions.
FIG. 7 shows a part of an operation waveform of a related-art pseudo resonance-type ringing choke converter. In FIG. 7, (a) shows a waveform of current in the switching element Q1, (b) shows a waveform of current flowing in the secondary side diode D1 and (c) shows a waveform between a drain and a source of the switching element Q1.
A time period of t1 to t2 indicates an on state of the switching element Q1. A time period of t2 to t4 indicates an off state of the switching element Q1, wherein a time period of t2 to t3 indicates a period during which the accumulated energy of the transformer T is discharged from the secondary coil S and the current flows in the secondary side diode D1. A time period of t3 to t4 is a half period during which the transformer T is ringing.
Here, a period of the self-excited oscillation of the pseudo resonance-type ringing choke converter is expressed by a following equation (4).
Ton: switching on time
Toff: switching off time
Lp: inductance value of primary coil P
Vin: input voltage
Vo: output voltage
lppk: peak value of primary coil current
lspk: peak value of secondary coil current
Vf: forward voltage of secondary side rectification diode
Cqr: capacity of voltage resonance condenser
Cmos: capacity between main terminals of switching elementTon=(Lp/Vin)×lppk  (1)Toff=(Ls×lspk)/(Vo×Vf)  (2)Tlc=2×Π×√(Lp×(Cqr+Cmos))  (3)Period of ringing choke converter=Ton+Toff+Tlc/2  (4)
From the equation (1), it can be seen that when an input voltage is low, the time Ton is lengthened. In addition, from the equation (2), it can be seen that when an output voltage is low, the time Toff is lengthened.
Accordingly, the power supply starts up at a state in which the input voltage is low and the output voltage starts at 0 volt. As a result, the period is lengthened. In addition, a switching frequency of the pseudo resonance-type ringing choke converter that performs a self-excited oscillation is lowered to the audible frequency at the time of startup of the power supply. This is an intrinsic problem that cannot be solved by the means for setting an off time and lengthening the off time, as disclosed in the related art.
Further, in living devices, a noise that is harsh to the ear is generated at the time of startup of the power supply.